immature

adj

1. (archaic)Occurring before the proper time; untimely, premature (especially of death).

   “And thou also canst best account for the causes of her immature death […].”

2. Not fully formed or developed; not grown.
3. Childish in behavior; juvenile.

   “You're only young once, but you can be immature the rest of your life.”

   “The man was immature for throwing a tantrum.”

   “Wilhelm Stekel - As quoted in The Catcher in the Rye (1951) by J. D. Salinger. The mark of the immature man is that he wants to die nobly for a cause, while the mark of a mature man is that he wants to live humbly for one.”

noun

1. An immature member of a species.

   “There are many genera and even families of Brachypylina for which immatures are not yet known, and thus numerous examples of adult convergence and misclassification remain to be revealed: such is the case with Hypozetes.”

   “While on a walk the next morning I found what looked like a patch of old growth habitat - perhaps somewhere the fires had missed - and to my astonishment saw a female Red-lored Whistler accompanied by an immature.”